Serrated choke system for electromagnetic waveguide



Nov. 27, 1956 KIYO TOMIYASU SERRATED CHOKE SYSTEM FOR ELECTROMAGNETIC WAVEGUIDE 2 Sheets-Sheet 1 Filed Nov. 22, 1950 INVENTOR Y0 7bM/ YASU ATTORNE Nov. 27, 1956 KIYO TOMIYASU v 2,772,402

SERRATED CHOKE SYSTEM FOR ELECTROMAGNETIC WAVEGUIDE Filed Nov. 22, 1950 2 Sheets-Sheet. 2

ll IIIIIIIHIlHlllllllllllllIlllIllllIl IIHIII INVENTOR A7 Y0 7bM/ W450 BY ATTO R N EY United States Patent SERRATED CHOKE SYSTEM FOR ELECTRO- MAGNETIC WAVEGUIDE Kiyo Tomiyasu, Flushing, N. Y., assiguor to Sperry Rand Corporation, a corporation of Delaware Application November 22, 1950, Serial No. 197,063

'7 Claims. (Cl. 333-98) The present invention relates to microwave energy enolOsi-ng apparatus, and is panticularly concerned with an improved choke joint system for providing a substantial electric junction between conductive members spaced a small distance apart, and inhibiting energy leakage there from.

A basic object of the invention is to provide a choke system affording the above mentioned features, and substantially free from transmission of microwave energy away from the space between the two conductive members.

A further feature of the invention is the provision of a choke system suitable for incorporationin wave guides and resonators and variable transmission devices, phase shifters, and movable probe devices, for maintaining high efficiency and substantial completeness of enclosure of microwave energy, even where a small gap exists be tween adjacent edges of conductive surfaces as where one plate or surface is required to be movable relativewto another plate or surface.

In accordance with the present invention, a choke system of very high efficiency is constructed of two conductive members spaced apart by a dimension very small compared to one-quarter wavelength, at least one of the conductive members having a series of tines extending adjacent to but slightly spaced from the other conductive member, the tines being of length substantially equal to an odd multiple of a quanter wavelength, and preferably being substantially one-quarter wavelength long. The width and spacing of the tines are appreciably smaller than one-quarter wavelength, a plurality of tines being provided within the dimension corresponding to the length of each time. The other conductive member maybe a continuous conductive surface, or may be serrated, the continuous surface being usually preferred since it is. fully effective in cooperation with the tines of the first conductive member, and since it is in general easier to construct.

Selected embodiments of the presentinvention are illustrated in the drawings, wherein Fig. 1 shows a serrated choke provided along one corner of a rectangular wave guide;

Figs. 2 and 3 are perspective views of 'a longitudinally movable probe device incorporating serrated chokes in accordance with the present invention;

Fig. 4 is a phase shifting system incorporating serrated chokes according to the present invention;

Figs. 5, 6, 7 and 8 are views of a variable-transfer directional coupler apparatus incorporatingchoke units in accordance with the present invention, Fig. 5 being a side elevation, Pig. 6 being a cross-sectional view taken on the line d-ti in Fig. 5, Fig. 7 being an isometric view with portions broken away to show details of the choke, and 8 being a sectional view taken on th line lit3 in Fig. 6; and

Figs. 9, 10, ll and l2 are illustrations of various modifications of the serrated choke system, Figs. 12A and 12B showing the different positions obtainable where sets of 2,772,402 Patented Nov. 27, 1956 tines are employed in both conductive members of a serrated choke wherein longitudinal movement is in volved.

Referring now to Fig. 1, a wave guide 21 is shown eX- tending between a microwave energy source 22 and a load 23. The wave guide 21 includes relatively broad walls 24 and 25, spaced apart by a dimension which may be less than the width of wall 25, with transverse electric polarization between these walls. A lower wall 26 is conneoted at its edges to side walls 24 and 25, and an upper wall 27 is attached along the upper edge of side wall 25. The spacing between walls 26 and 27-tthe a dimension of the Wave guidemay be, of the order of one-half wavelength or larger for etlicient communication of energy from source 22 to load 23. The transverse dimension between walls 24 and 25, i. e. the width of the bottom wall 26, may be of the order of one-quarter wavelength, for example, but may be narrower or broader, as desired.

The upper well 27 is illustrated as slightly narrower than the bottom wall 26, so that a very narrow gap remains open between wall 27 and wall 24. This gap may be of the order of wave length wide. in the absence of any radiation blocking provision at this gap, a high intensity electric field would result thereacross during transmission of energy from source 22 toward load 23, and appreciable m'crowave energy would be radiated therefrom.

in accordance with the present invention, a choke systern is provided for inhibiting radiation through this iongitudinally extensive gap, and for providing operation of the wave guide system 21 just as though the left-hand edge of the. upper wa1l 2'7 were connected to wall 24. For this purpose, two adjacent conductive members 26 and are provided, member 29 being serrated in such a way as to form ,a series of teeth or tines such as tines 31 therein. Members 23 and 29 are spaced apart by a dimension which may conveniently correspond to the width of the gap between walls 24 and 27, and their extent in the crosssectional plane of the Wave guide system 21 is an odd number of quarter wavelengths, preferably, one-quarter wavelength. The widths of the tines 31 and the gaps between them may be of equal orders of magnitude, and preferably several tines are provided per onequarter wavelength along the conductive member 2.).

The serrated choke system 28, 29 behaves in the manner of a senies of Lecher wire systems, which, being onequarter wavelength long and being open circuited at their upper ends, provide substantially zero impedance or an effective cross-connection across their lower ends. In Fig. 1, each Lecher wire system comprises one of the tines 3i and an image tine thereof, the adjacent surface of the conductive member 23 being the conductive plane (mirror plane) referred :to in locating the image tine, a

and the transverse spacing between two conductors of the Lecher wire system being effectively double the spacing between the physical tine and the adjacent sunfiace of conductive member 23.

Microwave energy chokes for use in wave guide apparatus systems have heretofore been constructed of two continuous surfaces, with very close spacing therebetween, and usuallytwith a dimension of at least: one of the sur faces of a quarter wavelength or multiple thereof, in the direction from the wave guide gap :to the outer edge. These systems suffer the disadvantage, however, that the choke system may itself serve :as a parallel conductor pair for supporting microwave energy transmission in a longitudinal direction, so that an appreciable part of the microwave energy fed into the Wave guide may ulti- I sults in inadequate efficiency of the microwave trans-S mission system, and in many instances, causes otherwise disadvantageous effects due to scattered energy radiation. With the serrations as shown in Fig. 1, however, not only is extremely low impedance effected across the gap between walls 24- 'and 27, but also, propagation of microwave energy longitudinally in the choke itself as well as propagation directly outward is substantially prevented by the choke system 28, 2 9 with the serrations or parallel tines formed "therein.

Figs. 2-8 illustrate microwave energy transmission devices wherein the serrated choke system is particularly useful. A wave guide system involving a travelling probe energy extraction arrangement is shown in Figs. 2 and 3. This system includes wave guide sections 41, 43 and 45, connected in tandem, and arranged with end flanges 47 and 49 for connection to further apparatus units. For example, an energy source may be connected to flange 47, and flange 49 may be connected to one end of a long wave guide having a microwave energy utilization device connected to its opposite end. A travelling probe system 51 is provided for cooperation with the intermediate wave guide section 43. The travelling probe system 51 includes a conductive plate 53 with a coaxial line section 55 situated therein, coupling loop 57 being provided at the lower end of the coaxial line 55 for intercepting :a portion of the magnetic field in the wave guide section 43. A detector 59 [and radio frequency by-pass capacitor 61 and measuring instrument 63 may be provided in connection with the coaxial line 55, for providing an indication of intercepted energy intensity in wave guide section 43.

Mechanical guide elements 65, 66, 67 and 68 are attached to wave guide sections 41 and 45, and arranged to support the conductive plate '53 for longitudinal movement as indicated at 69, for the desired variation of longitudinal position of coupling loop 57. These guide elements 65, 66, 67 and 68 space the plate 53 from the upper edges of the side walls of wave guide section 43 by a very small dimension compared to a quarter wavelength, so that the plate 53 is free from direct contact with the side walls of the wave guide section.

Two choke systems are provided, one at each of the upper edges of the side walls of wave guide section 43. One such choke system involves a series of tines 71, and the other choke system involves a series of tines 73, the longitudinally movable plate 53 serving as the opposite conductive mem her of each of the two choke systems.

As will be readily apparent, the guided longitudinal movement of the plate 53 has no effect upon the operation of the chokes involving tines '71 and 73 in cooperation therewith, since for every working posit-ion of the plate 53, the configuration in the choke system remains unchanged.

Thus, sliding contact arrangements are entirely obviated in the present invention, and the travelling probe system 51 including plate '53 and the coaxial line system carried thereon is provided with maximum freedom of longitudinal movement. The choke systems insure that the wave guide section 43 remains effectively fully closed at all times, and accordingly, this wave guide section is made to operate at full efliciency.

A wave guide of variable phase velocity is illustrated in Fig. 4. This wave guide may be employed in a directive antenna system wherein a relatively sharp directive beam is produced, the direction of the beam being regularly swung to and fro by the regularmechanical variations of the .a dimension in the elongated wave :guide 81- employed'to supp-1y energy to a series of antennas 83 situated at regular intervals therealong.

For such a system to operate most efficiently, it is necessary to avoid direct physical contact between the relatively transversely movable portions of the wave guide 81, but at the same time, it is necessary to insure that minimum impedance prevail across the gaps thereof, and that microwave energy leakage therethrough be substantially prevented.

In accordance with the present invention, a microwave energy choke may be provided along each of two effective junction lines in the wall surfaces of the wave guide 81, one choke comprising upwardly extending tines 85, and the other choke involving downwardly extending tines '87. These two series of tines are closely spaced from conductive members 89 and 91, respectively, which amount substantially to upward and downward extensions, respectively, of the broad side walls of the wave guide 81.

The wave guide wall of which conductor 89 is an up ward extension is driven in transverse reciprocating motion, alternately increasing and decreasing the a dimension of the wave guide 81. The vertical extensions at 89 and '91 are such as to exceed the corresponding extensions of the tires 85 and 87, respectively, so that for all positions of the movable wave guide portion, the effective Lecher wire systems in accordance with the general theory of operation outlined above remain substantially equal in length to the tines 8'5 and 87, themselves.

In this arrangement, as in the structures of Figs. 1 and 2, energy propagation through the chokes is effectively inhibited, in the longitudinal direction as well as directly outward, and the electrical performance of the wave guide system is substantially as efficient and as complete in energy confinement as would be the case with fixed junctions along all corners of the wave guide system.

An application of the present invention to 'a variabletransfer directional coupler is illustrated in Figs. 5, 6, 7 and 8. This structure comprises a lower wave guide 111 and an upper wave guide 113 arranged with their adjacent narrow walls separated by a very narrow gap. A choke joint system in accordance with the present invention is provided external of the gap along a length appreciably greater than the length of the openingsin the wave guides l'll'and 113, and a system of mechanical bearings is provided for maintaining the proper close spacing between the wave guides 111 and 113 and permitting guided relative longitudinal motion.

The mechanical supporting system includes a pair of downwardly extending side plates 161 and 163 afiixed to the upper wave guide 113, these side plates being vertically disposed and being separated from each other by a dimension appreciably greater than the cross dimeir' sion of the wave guides 111 and 113. Near the lower edges of the plates 161 and 163 are provided V-grooves, to form portions of .ball races for the bearing system. Corresponding V-grooves are provided in members 165 and 167 attached to the sides of Wave guide 111, and further V-grooves for the outer ball races are provided in angle members 169 and 171 which include vertically extending sections parallel to and outwardly disposed from plates 161 and 163, respectively. Four series of hardened balls 173, 175, 177 and 179 are situated in the longitudinal ball races thus provided, for accurately maintaining the relative positions shown in the cross sectional view of Fig. 6 while permitting freedom of longitudinal movement.

Referring to Figs. 6, 7 and 8, a series of choke tines exemplified by tines 181 are provided alongside wave guide 111. The tines 181 are downwardly dependent from Wave guide 113, and are spaced by a very small dimension from the outer wall surface of wave guide 111. These tines are preferably substantially onequarter wavelength long or very slightly shorter than one-quarter wavelength, their width dimensions being appreciably smaller than their length and the spacing between successive tines being preferably smaller than their width.

A similarrow of tines 183 is provided at the opposite side of wave guides 1.11 and 113. The features of the variable transfer directional coupler per so are set forth and claimed in application Serial No. 197,064 of Kiyo Tomiyasu and Seymour Cohn, filed concurrently herewith, and, entitled Variable Transfer Directional Coupler for Microwave Energy.

If desired, a pointer 145 may be provided on side plate 163, and a scale of relative transfer values for cooperation therewith may be provided on the upper horizontal surface or on the exposed side surface of angle member 171, attached to wave guide 111.

The choke joint system 181, 183, by virtue of the high impedance between the open-circuit lower ends of the tines 181 and 183 and the outer wall surface of wave guide 111 adjacent thereto, produces substantially zero impedance between the adjacent edges of the side walls of wave guides 111 and 113. By the use of the separate tines rather than a longitudinally extensive sheet of material having the same extent of downward projection, propagation of energy in the choke region parallel to the longitudinal axes of the guides is substantially suppressed.

The theory of operation which at present is believed properly applicable to the variable transfer directional coupler system involves the consideration of two modes of wave energy propagation which prevail within the mutually adjacent wave guide portions whose interiors are exposed to each other through the longitudinally extensive openings in the respective guides. It is customary to design a rectangular wave guide for efiicient transmission of energy of a known frequency, the mode of transmission being a fundamental mode of the guide usually referred to as the TE1,u mode. This is the dominant transverse electric mode. The a dimension of the simple rectangular wave guide ordinarily is such as to prevent it from transmitting energy of this frequency in any of the known higher modes.

When two of such wave guides are juxtaposed with their narrower faces mutually adjacent, and the adjacent narrower walls are opened throughout an appreciable longitudinal extent, the effect is to substantially double the "a dimension of the wave guide portion. This wave guide portion is then capable of supporting energy in two modes of propagation, the first being the simple transverse electric mode TE1,0 described in connection with the basic wave guide section, and the second mode being the asymmetrical mode designated TE2,0.

- These two modes are propagated along the doubled wave guide section at different phase velocities. At the point of entrance of the energy into the doubled wave guide section, they are in phase in the lower guide 111, assuming this guide supplied with energy at its left-hand end. At this point, these components are in phase opposition in the upper wave guide. Farther along the doubled wave guide section, however, the two components approach cophasality in the upper guide, even as they approach phase opposition in the lower guide. If the openings are of sufficient length and are in full register, complete energy transfer takes place from the lower guide to the upper guide, with unidirectional propagation being maintained, away from the source. i

For a position of intermediate register, a partial trans fer of-the energy to the upper guide takes place, so that an output power division between the upper and lower guides is accomplished, with the directional feature being retained. The power transfer to the upper guide varies with the extent of register between the openings as a sine squared function.

The skirts or side plates 161 and 163 serve not only as part of the mechanical bearing system but also as enclosing shields for the tines.

With the multiple-tine choke systems of the present invention, even where the tines are exposed, radiation therefrom and reflections and inefficiency of the conduit system are almost totally overcome. But some very slight radiation could occur due to the currents in the tines, and shielding thereover such as that in Figs. 58 is therefore desirable, providing complete insurance against even weak intercoupling between adjacent devices wherein the serrated chokes are used.

The serrated chokes of the present invention provide the dual advantages of affording maximum efiiciency by virtue of the substantially complete confinement of energy, and of insuring the required freedom from appreciable impedance across the gaps between the adjacent conductive surfaces of the lower and upper wave guides.

Figs. 9, 10, 11 and 12 indicate a few of the varied arrangements in which the choke systems may be constructed. Fig. 9 shows the tines arranged to extend a shorter distance than the conductive member adjacent thereto, the length of the tines being determinative of the effective lengths of the Lecher wire equivalents. In this structure, longitudinal or transverse movement or both may be employed without any loss of efiiciency of the choke system.

Fig. 9 illustrates the tines as rectangular in cross-section, with their broad adjacent surfaces positioned parallel to the opposite conductive member at very close spacing therefrom, and with the spaces between. successive tines somewhat smaller than the widths of the tines themselves. Such an arrangement is entirely workable, as is also an arrangement wherein the tines are of square cross-section. The widths of the tines, whether rectangular or square. may be equal to the spacings between successive ones thereof, or may be somewhat greater.

Fig. 10 illustrates a choke system embodiment wherein the tines are of elliptical crosssection. Such a construction may be used if desired, particularly where relatively high power capacity is desired, this system being less likely to be disturbed by voltage breakdown than an arrangement with prominent tine edges as illustrated in Fig". 9. For maximum power capacity, the ends of the elliptical tines may be rounded, also.

The tines may be made with circular cross-section, if desired, as illustrated in Fig. i], the circular tines being adjacent to a continuous sheet conductive member as shown therein. Yet a further variation of this invention may be provided as illustrated in Fig. 12A and 123, by having each of the two conductive members of the choke system arranged with serrations, i. e., provided with a series of tines. in this case each Lecher wire system refererd to for analysis of the choke system employs two tangible conductors, the inter-conductor spacing being equal to the spacing between upper and lower conductive members of the choke.

Where longitudinal wave guide movement is provided (i. e. in the direction transverse the tines), the configuration progresses gradually from the situation with paired tines as illustrated in Fig. 12A to the situation with staggered tines, as illustrated in Fig. 123. Even here, however, full choke operation is maintained, each Lecher system in this instance comprising one tine of one conductive member and the two nearest tines of the opposite conductive member. Accordingly, as with all of the preceding forms, longitudinal propagation of the microwave energy in the choke itself is substantially prevented, as is also the escape of energy transversely outward from the wave guide interioim While at least one of the two conductive members comprising the choke system of the present invention has been described as a serrated member, it will be readily apparent that this member with its several tines may be a unitary plate in which a series of saw cuts have been made, with or without further machining operations, but may. alternatively be a composite conductive member, as for example, longitudinal conductive bar in which are drilled a series of holes, with a series of wires or bars insertedtherein as the prongs or tines of the conductive member. i

While all of the tines shown in the drawings have been illustrated. as of uniform cross-section, the present invention is not limited to such conditions, but may involve tines which are not uniform, such as tapered tines, or tineswhich are rectangular at their bases and rounded at their outer ends, as examples.

The present invention is not restricted to use with rectangular wave guides operating in the TE1,0 mode (the dominant wave guide mode). t is readily usable for other transmission modes or other wave guide configurations including multi-conductor shielded systems as well, and is fully effective as to all electric field components across the gap.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

i. In a Wave guide system for transmission of micr wave energy of a. predetermined wavelength, a first conductive member and an adjacent second conductive member, said members being spaced apart by an elongated gap appreciably narrower than one-quarter wavelength, choke apparatus for providing very low impedance across said elongated gap, comprising a series of self-supported conductive tines on one of said members, the lengths of each of said tines being substantially equal to one-quarter wavelength, and the spacing between adjacent tines and the cross-sectional dimensions of each tine being appreciably less than one-quarter wavelength, said tines being conductively joined together at their base, each of said tines extending longitudinally in a direction substantially parallel to the adjacent surface of said other conductive member and being spaced apart therefrom by an air space appreciably less than one-quarter wavelength, the longitudinal direction of each of said tines being substantially perpendicular to the direction of propagation of energy in said wave guide system, said conductive tines cooperating with said other conductive member to minimize the escape of energy through said gap.

2. in a wave guide system for transmission of microwave energy of a predetermined wavelength, at first conductive member and an adjacent second conductive member, said members being spaced apart by an elongated gap appreciably narrower than one-quarter Wavelength, choke apparatus for providing very low impedance across said elongated gap, comprising a series of self-supported conductive tines on one of said members, the lengths of each of said tines being substantially equal to one-quarter wavelength, and the spacing between adjacent tines and the cross-sectional dimensions of each tine being appreciably less than one-quarter wavelength, said tines being conductively joined together at their base, each of said tines extending longitudinally in a direction substantially parallel to the adjacent surface of said other conductive member and being spaced apart therefrom by an air space appreciably less than one-quarter wavelength, the longitudinal direction or" each of said tines being substantially perpendicular to the direction of propagation of enegy in said wave guide system, said conductive tines cooperatappreciably narrower than one-quarter wavelength, choke apparatus for providing very low impedance across said elongated gap, comprising a series of self-supported conductive tineson one of said members, the lengths of each of said tines being substantially equal to one-quarter Wavelength, and the spacing betweenadjacent tines and the cross-sectional dimensions of each tine being appreciably less than one quarter Wavelength, said tines being conductively joined together at their base, each of said tines extending longitudinally in a direction substantially parallel to the adjacent surface of said other Conductive memberand being spaced apart therefrom by an air space appreciably less than one-quarter wavelength, the longitudinal direction of each of said tines being substantially perpendicular to the direction of propagation of energy in said wave guide system, said conductive tines cooperating with said other conductive member to minimize the escape of energy through said gap, and means fixed to' said first member for guiding said second member in relative movement adjacent to said first member and keeping said members substantially uniformly spaced apart at said gap, said guiding means further enclosing said series of tines for shielding the microwave energy therein.

4. The combination comprising a section of rectangular wave guide for propagating electromagnetic energy, said wave guide section having an opening through a side Wall thereof, conductive surface means situated adjacent said opening and being spaced apart from said side Wall by an air gap appreciably narrower than one-quarter Wavelength, said conductive surface means being movable relative to said side wall without appreciable variation in the widthof said air gap, said conductive surface means haying an aperture therein through which a portion of the energy within said wave guide may be coupled to an external load, and means for preventing the undesired escape of energy through said air gap including a series of selfsupported conductive tine means joined to said wave guide adjacent the edge of said air gap, the length of each of said tines being substantially equal to one-quarter wavelength of the energy to be propagated through said wave guide, the spacing between and the cross-sectional dimensions of said tines being appreciably less than one-quarter wavelength, said series of tines lying in a plane substantially parallel to said conductive surface means and being spaced apart therefrom by an air space appreciably less than one-quarter wavelength, each of said tines extending longitudinally in a direction substantially perpendicular to the direction of propagation of energy in said Wave guide, said series of tines and said conductive surface means cooperating. to provide a low impedance path across said air gap to prevent the escape of energy therethrough. 7

5. The combination comprising a section of rectangular Wave guide having an elongated opening through a side wall thereof, said opening extending substantially parallel to the longitudinal axis of said section of wave guide,

choke means for preventing the escape of energy through said elongated opening comprising a series of self-supported conductive tine means joined at their base to said wave guide section adjacent the edge of said elongated opening, the length of each of said tines being substantially' equal to one-quarter wavelength of the energy to be propagated through said wave guide, the spacing between and thecross-sectional dimensions of said tines being appreciably less than one-quarter wavelength, each of said tines extending longitudinally in a direction substantially perpendicular to the direction of propagation of energy in said wave guide, anda conductive surface situated ad jaceut said elongated opening, each of said tines extend ing longitudinally in a direction substantially parallel to said conductive surface and being spaced apart therefrom by an airspace appreciably less than one quarter wavelength, said conductive surface being movable relative to said series of conductive tines without appreciable variation in the width of said air space, said series of tines and said conductive surface cooperating to' provide a low impedance path across said elongatedopening to prevent a, second member comprising a plurality of parallel coplanar conductive tines, said second member being disposed outwardly of said wave guidesection, the planes of said first and second members being disposed parallel.

to the axis of said wave guide section, said first'and second members being mutually disposed with their planes substantially parallel and spaced apart a distance substantially equal to the width of said slit, the length of each of said tines being substantially equal to a quarter-wavelength and the spacing between adjacent tines and the cross sectional dimensions of each tine being appreciably less than a quarter wavelength, each of said tines extending longitudinally in a direction substantially perpendicular to said wave guide axis, at least one of said members being conductively affixed to said wave guide section along one edge of said slit.

7. In combination a wave guide section having a narrow longitudinal slit, a substantially plane conductive first member disposed outwardly of said wave guide section, a second member comprising a plurality of parallel coplanar conductive tines, said second member being disposed outwardly of said wave guide section, the planes of said first and second members being disposed parallel to the axis of said Wave guide section, said first and second members being mutually disposed with their planes substantially parallel and spaced apart a distance substantially equal to the width of said slit, the length of each of said tines being substantially equal to a quarter wavelength and the spacing between adjacent tines and the cross sectional dimensions of each tine being appreciably less than a quarter wavelength, each of said tines extending longitudinally in a direction substantially perpendicular to said wave guide axis, one of said members being conductively afiixcd to said wave guide section along one edge of said slit and the other of said members being movable with respect to said wave guide section in a direction parallel to the plane of said other member.

References Cited in the file of this patent UNITED STATES PATENTS 2,418,809 Albersheim Apr. 15, 1947 2,446,863 Yevick Aug, 10, 1948 2,451,876 Salisbury Oct. 19, 1948 2,521,844 Gordy Sept. 12, 1950 2,557,261 Collard June 19, 1951 2,572,628 Kock Oct. 23, 1951 2,595,186 Breetz Apr. 29, 1952 2,704,327 Chandler Mar. 15, 1955 FOREIGN PATENTS 572,881 Great Britain Oct. 26, 1945 

